1. Field of the Invention
This invention relates to weapon systems employing a liquid propellant, and particularly to such systems wherein the propellant is progressively combusted aft of the projectile as the projectile advances along the firing bore, i.e. a traveling charge system.
This invention also relates to such a system utilizing an initial source of combustion gas to provide an initial acceleration to the projectile and its traveling charge.
2. Prior Art
The classical propulsion of a projectile within the bore of a gun barrel is limited in velocity by the need to accelerate the combustion gases to the velocity of the projectile. This results in an increasingly large fraction of the thermodynamic expansion work being expended on accelerating the combustion gases. Normal ballistic models increase the apparent mass of the projectile by one-third the mass of the propellant. This assumption accounts for the kinetic energy imparted to the gases. For typical guns, the kinetic energy of the gases only amounts to about 10% at a velocity of 1000 m/sec. At 2000 m./sec. the fraction increases to approximately 50%. As the velocity approaches 3,000 m./sec. the gas kinetic energy approaches 100% (nothing left for the projectile.) This effect produces what is called the "limit velocity" beyond which a conventional gun propulsion system cannot operate. The Traveling Charge Propulsion system provides a theoretical means around this limit.
As shown in FIGS. 1 and 2, in a traveling charge propulsion system, part or all of the charge C travels down the bore of the gun barrel with the projectile P. Propulsion occurs by the rapid combustion of the charge in the rear portion of the charge, sometimes called "cigarette burning". The reference frame shown in FIG. 1 is taken as moving with the projectile P, wherein:
A.sub.BORE =cross-sectional area of the bore PA1 L.sub.cp =length of charge of propellant PA1 .rho..sub.p =density of the propellant PA1 .rho..sub.g =density of the combustion gas PA1 A=acceleration of the projectile PA1 M=burn rate of the propellant [slugs/sec] PA1 P.sub.BASE =pressure at the base of the projectile PA1 P.sub.L =pressure at the interface of the propellant and the combustion gas PA1 P.sub.w =pressure at the exit of the combustion zone PA1 r=linear burn rate of the propellant PA1 V.sub.j =exhaust velocity of the combustion gas at the exit of the combustion zone.
The accelerating force on the projectile and the traveling charge is made up of two terms. The first term can referred to as the "pressure" term, where the combustion of the charge produces an elevated pressure at the exit of the combustion zone. The second term can be referred to as the "thrust" term, where the thrust is the result of the momentum of the combustion gas exiting the combustion zone: ##EQU1##
Both of these terms increase as the rate of combustion increases. The total thrust divided by the mass consumption rate is referred to as the "specific impulse" (a rocket It can be shown that this parameter is a maximum when the gas velocity is greatest. Since this combustion is taking place in a constant area duct (Rayleigh flow) the maximum velocity is the sonic velocity. Under these conditions, typically 200 pounds of total thrust is generated for each pound of propellant consumed per second. For a 30mm weapon to produce 50,000 lbs. of thrust, a consumption rate of 250 lb./sec. is required. This consumption rate requires a linear burn rate of approximately 300 ft./sec. Since normal solid propellants only burn at approximately 1 foot per second at gun pressures, it is apparent why the concept of solid propellant traveling charge propulsion has yet to be made workable.
The use of liquid propellant for a traveling charge system has been proposed previously.
In U.S. Pat. No. 4,011,817, issued Mar. 15, 1977, E. Ashley disclosed a system which utilized the difference in density between the combustion gases and the charge of liquid propellant as the source of energy for the injection of propellant into the combustion chamber. A primer provided the initial acceleration of a cavity generator. A charge of liquid propellant aft of the projectile flowed relatively aftwardly past the cavity generator into the combustion chamber which was formed by and was aft of the cavity generator. The velocity provided by the primer was in the order of hundreds of feet per second.
In U.S. Ser. No. 255,065 filed Apr. 3, 1981, M. J. Bulman disclosed another system which utilized liquid propellant to provide a traveling charge to a projectile.
The major drawback to the liquid propellant bulk loaded approach as disclosed, for example, in U.S. Pat. No. 4,085,653, issued to D. P. Tassie et al on Apr. 25, 1978, is poor control over combustion. The combustion in a bulk loaded gun is largely the result of the growth of fluid dynamic instabilities. A large burning rate is required before there is any acceleration of the projectile and this amplifies any variations in the ignition system.
FIG. 3A shows a typical bulk loaded liquid propellant Gun prior to ignition. The cylindrical chamber is completely filled with liquid propellant. The forward end of the chamber is closed by the base of the projectile. The projectile is seated in the forcing cone of the barrel. The rear of the chamber is closed by a bolt containing the igniter. When the igniter is energized, a jet of hot gases emerges from the igniter vent (see FIG. 3B). This jet, as it enters the chamber must displace propellant in the chamber. Since the chamber is initially constant in volume, this displaced propellant must compress the remaining liquid. Even a small compression will produce a large pressure rise in the liquid. For example, if the igniter jet occupies 1% of the chamber volume, a pressure rise of several thousand pounds per square inch results. Ignition of the main charge of liquid propellant occurs on the surface of this expanding bubble of hot igniter gases. The projectile starts moving when the gas bubble has grown to no more than a few percent of the chamber volume with a nominal surface area which is less than the area of the base of the projectile. In order to sustain a rising pressure in the face of the rapid acceleration of the projectile, the actual burning surface must be 100-1000 times the nominal value. This is achieved in the bulk loaded cycle by the violent interaction between the igniter jet and the liquid propellant. The shearing of the liquid surface by the penetration of the igniter jet produces a rough surface akin to ocean waves on a windy day (the Helmholtz instability--see FIGS. 3C and 3D). If insufficient surface area is generated, projectile forward motion will result in a declining pressure and very poor performance. If too much surface area is generated, dangerously high levels of pressure will occur. Since the surface area generation is the result of great amplification in these fluid mechanical instabilities, slight variations in any part of the process will have a major impact on the pressures generated.
To illustrate the sensitivity to variations in the process, it can be shown that combustion of only 1% of the charge before projectile forward motion can produce a pressure rise in excess of 100,000 PSI (which is often seen). FIG. 4 shows a typical bulk loaded pressure time curve.
Accordingly, it is an object of this invention to provide a bulk loaded, liquid propellant gun system having controlled ignition and combustion which provide an improved traveling charge to propel the projectile.
Another object is to provide a liquid propellant gun system with an improved control over ignition and combustion which avoids the strong feedback present in the conventional bulk loaded cycle.
A feature of this invention is the provision of a liquid propellant gun system having a traveling charge which is ignited after both such charge and the projectile have been accelerated forwardly.